[Gluster-devel] ZkFarmer

[Jeff Darcy]

I've long felt that our ways of dealing with cluster membership and staging of
config changes is not quite as robust and scalable as we might want.
Accordingly, I spent a bit of time a couple of weeks ago looking into the
possibility of using ZooKeeper to do some of this stuff.  Yeah, it brings in a
heavy Java dependency, but when I looked at some lighter-weight alternatives
they all seemed to be lacking in more important ways.  Basically the idea was
to do this:

* Set up the first N (e.g. N=3) nodes in our cluster as ZooKeeper servers, or
point everyone at an existing ZooKeeper cluster.

* Use ZK ephemeral nodes as a way to track cluster membership ("peer probe"
merely updates ZK, and "peer status" merely reads from it).

* Store config information in ZK *once* instead of regenerating volfiles etc.
on every node (and dealing with the ugly cases where a node was down when the
config change happened).

* Set watches on ZK nodes to be notified when config changes happen, and
respond appropriately.

I eventually ran out of time and moved on to other things, but this or
something like it (e.g. using Riak Core) still seems like a better approach
than what we have.  In that context, it looks like ZkFarmer[1] might be a big
help.  AFAICT someone else was trying to solve almost exactly the same kind of
server/config problem that we have, and wrapped their solution into a library.
 Is this a direction other devs might be interested in pursuing some day,
if/when time allows?


[1] https://github.com/rs/zkfarmer